460 Mr. Yarrell on the Beak of the Crossbill. 



To those who have not made the habits and economy of birds an object 

 of investigation, it may be necessary to state that our two species of 

 Crossbill are the only British birds that exhibit or seem to require any 

 lateral motion of the mandibles, and it is the object of this paper to 

 describe the bony structure and muscles by which this peculiar and 

 powerful action is obtained. 



It may be necessary to premise that Mr. Townson in his " Observations 

 " in Natural History and Physiology," has devoted a chapter to the struc- 

 ture of the bill of the Crossbeak, some extracts from which will be 

 introduced in this notice ; but to show that I am not occupying the valu- 

 able pages of the Journal with a tale twice told, the first sentence I 

 select is as follows. Speaking of the beak, he says, " I ought now to 

 " give the anatomy of it, and I wish it were in my power; but on the 

 " muscles, by which the motions I have spoken of are performed, I find 

 " scarce any memorandums in my portfolio." 



The beak of the Crossbill is altogther unique in its form ; the mandi- 

 bles do not lye upon each other with their lateral edges in opposition as in 

 other birds, but curve to the right and left, and always in opposite direc- 

 tions to each other. In some specimens the upper mandible is turned to 

 the right, the lower mandible curved to the left; in others, the position 

 of the mandibles is reversed as to their direction. In the specimen I 

 examined the upper mandible curved downwards and to the left, the 

 under portion turned upwards and to the right, as the figures 1 and 2, 

 plate xiv, will demonstrate. When holding the head of this bird in 

 my fingers, I found I could bring the point of the under mandible in a 

 line underneath and touching the point of the upper, but not beyond 

 it towards the left side, while on its own side the point passed with ease 

 to the distance of f of an inch. The upper mandible has a limited 

 degree of motion on the cranium, the superior maxillary and nasal bones 

 being united to the frontal by flexible bony laminae. 



The form as well as the magnitude of the processes of some of the 

 bones of the head are also peculiar to this bird. 



The pterygoid processes of the palatine bones are considerably elon- 

 gated downwards, as shewn at figure 3, letter a, to afford space for the 

 insertion of large pterygoid muscles. The os omoideum on each side 

 {figure 3, letter ft,) is strongly articulated to the os quadratura, (figure 3, 



